The invention relates to alloys that can be used as substrates for superconductors, to superconductors including such substrates, and to methods of making these alloys and superconductors.
Superconductors, including oxide superconductors, are used in a variety of applications. Some superconductors can demonstrate limited mechanical strength. Often, the mechanical strength of a superconductor can be enhanced by forming a multilayer article that includes a layer of superconductor material and a substrate layer, but the substrate should exhibit certain properties.
The substrate should have a low Curie temperature so that the substrate is not ferromagnetic below the superconductor""s critical temperature. Furthermore, chemical species within the substrate should not be able to diffuse into the layer of superconductor material, and the coefficient of thermal expansion of the substrate should be about the same as the superconductor material. Moreover, if the substrate is used for an oxide superconductor, the substrate material should be relatively resistant to oxidation.
For some materials, such as YBa2Cu3Ox (YBCO), the ability of the material to act as a superconductor depends upon the crystallographic orientation of the material. For these superconductors, the substrate should have a crystallographic orientation that allows the material to act as a superconductor. Often, good superconducting properties are observed in these materials when the substrate has a biaxially textured surface. One type of biaxial texture is cube texture, in which the lattice is oriented such that the cube face is parallel to the surface. In addition, the cube edge in each crystallite is parallel to the cube edge in all neighboring crystallites. Examples of cube textured surfaces include the (100)[001] and (100)[011] surfaces, and an example of a biaxially textured surface is the (113)[211] surface.
Some substrates do not readily meet all these requirements, so one or more buffer layers can be disposed between the substrate and the superconductor layer. The buffer layer(s) can be comparatively resistant to oxidation, and reduce the diffusion of chemical species between the substrate and the superconductor layer. Moreover, the buffer layer(s) can have a coefficient of thermal expansion and a crystallographic orientation that is well matched with the supercondutor material.
Buffer layers are commonly formed using epitaxy. An epitaxial layer is a layer of material that is grown on a surface such that the crystallographic orientation of the layer of material is determined by the lattice structure of the surface on which the layer is grown. For example, for an epitaxial buffer layer grown on the surface of a substrate layer, the crystallographic orientation of the epitaxial layer is determined by the lattice structure of the surface of the substrate layer. Techniques used to grow epitaxial buffer layers include chemical vapor deposition and physical vapor deposition.
Some pure metals, such as copper and nickel, can be prepared to have a desirable crystallographic orientation (e.g, a biaxial texture or cube texture) by a process that involves first rolling the metal, and then annealing the metal. However, these pure metals may exhibit certain properties that are inappropriate for a superconductor supporting substrate. For example, nickel has a relatively high Curie temperature, and copper is relatively easily oxidized.
Attempts have been made to provide substrates for superconductors that are crystallographically oriented alloys. These substrates have been formed by first rolling and annealing a metal, then diffusing a different metal into the pure metal to form the alloy. This can result in a nonhomogeneous alloy.
The invention relates to alloys that can be used as substrates for superconductors, to superconductors including such substrates, and to methods of making these alloys and superconductors. The alloys can exhibit a variety of advantages, including good oxidation resistance, low Curie temperature, good homogeneity, and/or good surface texture.
In one aspect, the invention features an alloy having a biaxially textured surface. The alloy includes a first metal, a second metal and at least about 0.5 atomic percent of an oxide former. The first metal is different than the second metal, and the oxide former is different than the first and second metals. The alloy can be made by a process that includes rolling the alloy, and then annealing the alloy.
An xe2x80x9coxide formerxe2x80x9d as used herein, refers to a metal that tends to form oxides that are more stable, both kinetically and thermodynamically, than Cu or Ni oxides. Aluminum (Al) is a preferred oxide former.
In another aspect, the invention features an alloy that includes a first metal, a second metal and at least about 0.5 atomic percent of an oxide former. The alloy has a native oxide exterior with a biaxially textured surface. The native oxide is formed of an oxide of the oxide former. The second metal is different than the first metal, and the oxide former is different than the first and second metals. The alloy can be made by a process that includes rolling the alloy, and then annealing the alloy.
In a further aspect, the invention features an article including an alloy and an oxide layer disposed on a surface of the alloy. The alloy undergoes substantially no oxidation when the article is exposed to an atmosphere containing 1% oxygen at 900xc2x0 C. for at least two hours.
In yet another aspect, the invention features an alloy with a biaxially textured surface. The alloy includes copper and from about 25 atomic percent nickel to about 55 atomic percent nickel. At least about 65 volume percent of the alloy is formed of grains having a biaxial texture. The alloy can be made by a process that includes rolling the alloy, and then annealing the alloy.
The alloys preferably have a Curie temperature of less than about 80 K (e.g., less than about 40 K or less than about 20 K).
The alloys can contain more than one oxide former.
The alloys can be homogeneous alloys.
The alloys can be relatively resistant to oxidation.
The alloys can have a surface that is biaxially textured or cube textured.